1. Field of the Invention
This invention relates to a method of firmly bonding metal and ceramics, prosthetic teeth produced by the application of such method and composite plating materials used in such bonding method.
2. Description of the Prior Art
In the past, ceramics were directly coated on the surface of metal and then fired to fuse them. However, because this method attained only poor bonding strength, another method is generally employed in which the surface of the metal is first roughened by etching. Then the ceramic is fired onto the roughened surface so as to cause the ceramic to base. In this manner one is able to obtain higher bonding strength. This method also has disadvantages in that sufficiently high bonding strength can not be obtained because of the great difference in the physical properties between metal and ceramics, and further because the ceramics can be separated from their bonded position by small stresses.
Accordingly, these conventional methods cannot be used when the bonded products are thereafter subjected to severe vibration, repeated heating and cooling or excessive load. Thus, these methods have had only restricted fields of application and even then are not reliable.
A method of producing prosthetic teeth, especially cast metal crowns fused to porcelain, which is conventionally used in the dental field, comprises coating cast metal crowns with opaque porcelain and then with dental crown colored porcelain, in order to hide the color of the metal and to improve the bonding to the dental crown colored porcelain in the upper layer. The coated crown is then fired. However, this method also has the problem of weak bonding of the metal crowns to opaque porcelain. One is now also forced to use a nickel-chromium alloy due to a rise in prices and a shortage of non-oxidizable noble metals such as gold or palladium which are ideal as a material for the cast metal crowns. When the opaque porcelain coated on a nickel-chromium alloy is fired at a temperature ranging from 940.degree. C. to 960.degree. C., the alloy is oxidized and gas is evolved. It is believed that the gas remains within the opaque porcelain such that firm bonding can not be obtained and defects such as break-away and discoloration of the opaque porcelain occur.
In order to eliminate these defects, a non-oxidizable metal such as gold, palladium, nickel, chromium and the like could be plated on the surface of the cast metal crown of nickel-cromium alloy. Only by plating the non-oxidizable metal, however, can the surface of the cast metal crown be prevented from the oxidation. In such a method, however, the inorganic opaque porcelain is not firmly bonded.
Thus, it is an object of this invention to provide a novel method of firmly bonding metal and ceramics.
It is another object of this invention to provide composite plating materials used for firmly bonding metal and ceramics.
It is still another object of this invention to provide prosthetic teeth in which porcelain is firmly bonded to metal crowns such that it cannot be easily broken away and/or discolored.